1. Field of the Invention
The present invention relates to a process for forming an image by developing a direct positive silver halide photographic light-sensitive material to obtain a direct positive image.
2. Description of the Prior Art
Photographic methods for obtaining a direct positive image without need of a reversal process step or a negative film have been well known.
Known methods for forming a positive image by using a direct positive silver halide photographic light-sensitive material can be classified mainly into two types considering practical usefulness.
One is a method for obtaining a direct positive image by using pre-fogged silver halide emulsions and destroying the fog nucleus of exposed areas (a latent image) by utilizing solarization, Harschel effects or the like.
The other is a method for obtaining a direct positive image by imagewise exposing to light photographic materials using internal latent image type silver halide emulsions which have not previously been fogged followed by surface development during or after fogging process. In this connection, the above-mentioned internal latent image type silver halide photographic emulsion means a silver halide photographic emulsion which has light-sensitive nucleus mainly inside the silver halide grains and by exposure of which emulsion a latent image is formed mainly inside the grains.
Methods of the latter type generally have higher sensitivity than those of the former type, and are suitable for uses wherein high sensitivity is required. The present invention relates to the latter type method.
Various techniques have hitherto been known in this technical field. Main such techniques and disclosed, for example, in each specification of U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and U.K. Pat. Nos. 1,151,363, 1,150,553 and 1,011,062.
Photographic light-sensitive materials of relatively high sensitivity as direct positive type can be prepared by using these known methods.
Further, the mechanism of direct positive image formation is disclosed in detail, for example, in T. H. James, The Theory of The Photographic Process vol. 4, chapter 7, pages 182 to 193, U.S. Pat. No. 3,761,276 and the like. According to the disclosures, it is believed that fog nuclei are formed selectively on the surface of the silver halide grains of non-exposed areas based on a surface desensitizing action owing to internal latent images formed by the first imagewise exposure, and then a photographic image (direct positive image) is formed on the non-exposed areas by conducting a usual so-called surface developing process.
In the above-mentioned, as methods for selectively forming fog nuclei, a method generally called "light fogging method" wherein the second exposure is given on the whole surface of the light-sensitive layers (for example, U.K. Pat. No. 1,151,363) and a method generally called "chemically fogging method" wherein nucleating agents are used have been known. The latter method is disclosed, for example, Research Disclosure, vol. 151, No. 15,162 (published in November, 1976), pages 76 to 78.
Light fogging methods have disadvantages in that properties of a finished product of a light-sensitive material are liable to change depending upon change in exposure amount, development time, developing solution components, processing temperature and the like.
On the other hand, chemically fogging methods have disadvantages in that the methods must be carried out in high pH conditions which lead to deterioration of a developing agent by air oxidation, which in turn leads to reduction of fogging effect.
Japanese Published Examined Patent Application No. 22515/1971 discloses a method for forming a direct positive color image by using low intensity whole surface exposure and a nucleating agent together, but there is no specific disclosure about the nucleating agent.